Research outlined in this proposal focuses on the anatomy and pharmacology of vocal control pathways of the songbird brain. Work to be done falls into three categories. 1. We shall continue to use horseradish peroxidase and radioactively labelled amino acids to describe in detail the afferent inputs to vocal control stations. To this end we will employ electrophysiologically aided targetting of small nuclei and transneuronal transport. As a complement to these anatomical studies, lesion experiments will explore the role of various afferent nuclei in song learning and in maintenance of learned song. 2. We will use computer-aided descriptions of Golgi stained neurons in nuclei such as HVc, RA and nXIIts known to mediate vocal control to characterize differences between these stations in the right and left hemisphere and differences between the sexes. This will be followed by a number of experiments involving surgery, behavioral manipulation and control over testosterone levels in an attempt to identify microanatomical properties of neurons in HVc, RA and nXIIts that change as a result of use, hormone, and learning. 3. We will also conduct a series of pharmacological studies to identify the way in which testosterone affects AChE levels in the syringeal neuromuscular synapse. The latter system has been chosen as a convenient place to start to characterize the anatomical and pharmacological effects of Testosterone on synapses. After we have been able to identify all brain stations necessary for vocal learning, and after we have been able to assign anatomical and physiological variables related to sex, age, use and hormones, we hope that there will be a remainder of anatomical and physiological properties or events that can be related in a causal manner to the learning of a complex motor task.